This invention relates to circuit interconnections and, more particularly, to multiple-conductor circuit interconnections in which a pattern of connection pads is provided on a substrate such as a printed circuit board, especially where there is a lack of precise dimensional control as is typically the case with flexible printed circuit substrates.
A particular application of the invention is the manufacture of ultrasound probes for medical applications. As the number of transducer elements in typical ultrasound probes increases and the desired size of the probe handle and other packaging decreases, there is a need to increase the density of electrical connections between the transducer elements and the probe cable. For reasons of manufacturability and economics, the transducer pallet and cable are usually built and tested as separate subassemblies, then joined. If the pallet and cable each terminate in a flexible printed circuit, then the preferred joint is a flex-to-flex bond. Such a bond comprises one or more rows of metallized connection pads on the transducer pallet flex circuit, similar row(s) of pads on the cable flex circuit, and an anisotropic conductive adhesive which, under heat and pressure, forms an electrical and mechanical bond between corresponding connection pads of the two flex circuits.
Flexible printed circuit substrates are typically made of a polyimide, such as Kapton.RTM. (a DuPont product), with a typical thickness within the range of 25 to 75 microns. Another flexible printed circuit substrate material conventionally employed is polyester.
A factor that limits the achievable density of a flex-to-flex bond is the need to allow for variations in the pad pitch between the two flexible printed circuits. Typically there is a lack of precise dimensional control during manufacture of flexible printed circuits. Dimensional variations, due to temperature, humidity, flex circuit shrinkage, and other manufacturing variables, constrain both the minimum pad width (to achieve electrical contact at all pads) and the minimum space between pads (to avoid short circuits). Design of a flex-to-flex bond becomes a tradeoff between desire for high density and good yield, and the cost of tight tolerances for the flexible printed circuits.
Electrical connection bonds between two flex circuits or between a flex circuit and a rigid circuit board are used in various situations and products, not just during the manufacture of ultrasound arrays. A major application is electrical connection to flat panel (LCD) displays for appliances, computers, and aircraft. Another application is the fabrication of multilayer flex circuits by lamination of two or more single-layer circuits with anisotropic conductive adhesive. Yet another application is the mounting of integrated circuit packages to a circuit board substrate employing surface mount technology. Tradeoffs between density, yield, and cost occur in the design of all of these products.
Conventionally these problems are addressed by attempting to compensate for dimensional variations. The initial artwork defining the connection pads is made slightly over- or under-size to compensate for the anticipated shrinkage or swelling of the parts during manufacturing. A problem with this approach is that the shrinkage varies slightly from one process run to another or from one batch of material to another, making it nearly impossible to predetermine the exact shrinkage. In any event, the pitch of the interconnection pads must be large enough to accommodate the residual variation in the dimensions of the parts.